This invention relates to improved tantalum powders and to anodes prepared therefrom and specifically to powders that can be pressed without using a binder into anodes with high green strength before sintering and with excellent electrical characteristics after sintering. These powders also have good flow characteristics which are achieved without additives.
One common and well-known use of tantalum powder is for producing electrodes for either solid or liquid type electrolytic capacitors. Such electrodes are made by pressing the tantalum powder to form a coherent compact, sintering this compact, and subsequently forming a dielectric film on the sintered compact.
In such capacitors it is desired to have as high specific capacitance CV/g as possible. U.S. Pat. No. 3,418,106 to Pierret, dated Dec. 24, 1968, discloses an agglomerated tantalum powder, crushable as tantalum, which, when fabricated into an electrode, provides enhanced specific capacitance. The agglomerated tantalum powder described in this patent also has improved flow characteristics as compared to prior powders. The method used to produce such agglomerated tantalum powder is disclosed in U.S. Pat. No. 3,473,915 to Pierret, dated Oct. 21, l969. The two referenced Pierret patents describe generaly the nature of agglomerated tantalum powders.
Although such powders have reasonably good flow characteristics, to make these powders and other prior art powders dispensable in automatic pellet-making machines at high speed with good weight control from pellet to pellet, the capacitor manufacturers historically have added binders to the powders. The binders generally are a carbonaceous type, such as carbowax, arcrawax and glyptol. Such binders also were necessary to provide adequate "green" strength (a condition after forming but prior to sintering) in the pressed compact to withstand handling during pressing or loading for subsequent sintering. However, the use of carbonaceous binders is objectionable because a two-step sintering cycle (a dewaxing heat treatment followed by a higher temperature heat treatment is necessary. In addition, residual carbon remaining in the sintered parts tended to reduce the electrical qualitities.
In the manufacture of compacts using high speed automatic presses, a percentage of the powder is not pelletized due to intentional die overfill. In addition, some parts are defective as pressed, and are crushed to powder. The overfill powder and the powder recovered from such crushed compacts is recycled and ultimately can be pressed a second time, or a third time, or more. State-of-the-art powders tend to be degarded in quality or characteristics during such recycle.